Strength through shape

If they are intelligently engineered, lightweight materials can have three times the strength while maintaining the same material density. This is one of the results of a project on the properties of cellular materials at the Vienna University of Technology that is supported by the Austrian Science Fund.

The prerequisite for this discovery was a technique that made it possible to directly transform computer models into physical samples. In the future this project will concentrate on applying this technique to produce bone replacement materials.

Both nature and technology have long since been aware of the benefits of cellularly structured materials, namely a high degree of strength at low weight. That's why the research team of Jürgen Stampfl and Heinz Pettermann at the Department of Mechanical Engineering of the Vienna University of Technology have been studying the crucial effect that cell geometry has upon these benefits by examining how the choice of cell geometry augments material strength while maintaining the same weight.

From Computers to Lab Bench
The key factor for success was rapid prototyping, a technique that makes it possible to quickly transform virtual computer models of material structures into physical models. This is what Stampfl has to say about it: 'Rapid prototyping is a method that is just as fascinating as it is limited. We can design structures on the computer that have never existed anywhere in nature or technology. We then calculate how strong they are and test them directly in actual experiments. However, this production technology is based upon the method of stereolithography that restricts the range of available materials, and therefore the application of the models.'

For stereolithography, they start off by converting computer models into thin virtual slices. Then, a special software program uses those as a basis for controlling a series of micromirrors through which light is projected onto a light-sensitive layer of resin. Wherever light falls upon the resin, it hardens the material that was originally liquid. This is how structures can be created that have a high degree of resolution and whose mechanical properties can be subjected to direct experimental examination.

Something fascinating now happens with structures that have a simple basic cellular unit resembling a hollow cube with edges measuring 4 millimetres. Depending upon the basic unit's configuration, their stiffness and strength varied by a factor of three while maintaining the same specific weight. For instance, irregular structure as can be found in nature are not very strong, however they are substantially less sensitive to improper loading that might come about in something as a fall.

Bones from Computers
The next thing the project team is targeting will be developing bone replacement materials. As bone is one of the many natural materials built upon cells. The key subject of Stampfl's work is developing a variation of rapid prototyping for producing moulds later filled with a ceramic gel. After this gel hardens and the mould is removed, a ceramic structure with a defined pore size is obtained. Stampfl points out that 'we use hydroxyapatite, a basic building block of our bones, as the ceramic gel. It allows us to create a structure with arbitrary shape in order to use the material as scaffold suitable for growing bone-forming cells. Yet, we can't say today how or whether this substitute bone material might be used at all in real-life situations.' However, they are working together with the Max Planck Institute for Colloids and Interfaces (located in Golm, Germany) and the Ludwig Boltzmann Institute for Osteology (in Vienna) to demonstrate that osteoblasts, i.e. the cells that form bones, settle in this matrix and then survive over a longer period of time.

The President of the FWF, Professor Georg Wick, remarks that 'when we get results from a scientific discipline such as material science and they are applied in another such as medicine, that's what we call synergy. And that's what basic research is all about, which is why we focus on supporting it.'